Transistor amplifiers have a peak efficiency for a particular output power that is a function of geometry (i.e. circuit components and layout), load and supply voltage. In conventional radio frequency (RF) power amplification these characteristics are fixed based on the peak input level expected. For amplifiers presented with an input signal having a wide dynamic range, the input signal infrequently achieves peak levels and frequently operates below peak levels. Under such conditions, the amplifier may exhibit low overall efficiency.
Various techniques are known in the art for enhancing amplifier efficiency based on the supply voltage. One broad classification of solution is envelope tracking.
In a known envelope tracking technique, a switch mode pulse width variable modulator may be combined with a linear AC amplifier such that an efficient switch mode supply provides low frequency components of an output signal that contains a majority of the required power, and the linear AC amplifier provides a high bandwidth signal to provide the high frequency components of the output signal and correct errors in the switch supplied output. A power supply with high bandwidth and generally good efficiency is thereby provided. An example of an RF amplification stage incorporating a particularly advantageous technique in accordance with these principles is disclosed in British Patent No. 2398648.
The implementation of an RF amplification stage in accordance with the advantageous principles of the envelope tracking technique disclosed in the above-identified British patent results, for example, in improving an amplifier from 10% efficiency to 50% efficiency. It is desirable to look to improve the efficiency further.
A typical amplification stage includes a driver stage and an output stage. In an RF amplification stage the output stage typically comprises the RF amplifier, being the portion of the amplification stage at which high power signals are generated. The above-identified British patent teaches the introduction of an efficient envelope tracking power supply unit for providing the power supply voltage to the output stage of an amplification stage. This addresses inefficiencies in the amplification stage. However inefficiencies also exist in the driver stage, and improving the efficiency of the driver stage results in further improvement in the efficiency of the overall amplifier.